The present invention relates to a colour liquid crystal projection display system comprising a liquid crystal panel having an array of display elements for modulating light to produce a display output, illuminating means for ,directing three differently-coloured light illuminating beams onto the panel such that they are incident on the panel from mutually different directions, the panel being provided with a microlens array at its input side for directing the three different colours of input light onto respective sets of the display elements in the array, each microlens element overlying a group of three display elements, and a projection lens for collecting the display output from the panel and projecting the display output onto a screen.
A projection system of this kind is known from EP-A-0465171. In a described embodiment, a white light source, comprising for example a metal halide arc lamp, generates a beam of generally parallel white light which is directed onto a set of three dichroic mirrors whose planes are arranged at mutually different slant angles. These dichroic mirrors produce respectively three differently-coloured, red (R), green (G) and blue (B), beams of light which are directed towards the input side of the liquid crystal panel such that the three beams are incident upon the panel from three different directions, with the central beam arriving perpendicularly to the panel, and overlap at the region of the panel. The panel carries a microlens array at its input side. Each microlens element in the array overlies a group of three immediately adjacent LC display elements in a row of the display element array, constituting red (R), green (G) and blue (B) display elements, and serves to converge and direct light from each of the three, R, G and B, differently-coloured input light beams through a respective one of those three display elements. The display elements modulate the light passing therethrough in accordance with applied R, G, B video signals. The modulated light outputs from all the display elements in the array so illuminated are collected by a projection lens for projection onto the display screen.
This system has advantages over other known kinds of colour LC projection systems. In comparison with the kind of system which uses three separate LC panels each operable with a respective colour light, the number of components and complexity of the system is, of course, considerably reduced, although the display element density of the panel has to be three times as great to provide the same display resolution. In comparison with conventional colour LC projection systems using a single LC panel in which colour is obtained by using a red, green and blue colour microfilter array in association with the display elements in conjunction with an illuminating beam of white light, the light output, and hence brightness, is significantly increased for a given light source as around two thirds of the input light in the conventional system is absorbed or reflected by the colour filters.
However, the system described in EP-A-0465171 is not without its own problems and can suffer from the effects of stray light which lead to a display image with inferior colour purity and colour contrast being produced. It is mentioned in EP-A-0465171 that problems with stray light may be caused by the degree of parallelisation of the illuminating light being poor, so that the illuminating beams are incident on the panel from directions other than the predetermined directions. In such a case it is suggested that the light from the light source could be converged on spots by a condensing lens and unnecessary light cut out using a slit or a pin hole. Besides making the system more complex and more expensive, this would require more space and thus be unsuitable for a compact projection system.